What does this mean!!

"A const member function cannot call a non-const member function of the same class, since this would potentially modify the object."

Can anyone give an example of this? If it is declared a const class how any member function even be called if it isn't declared const? I am really confused. Could someone please help me at what exactly this means. I know about why it needs to be const because of the 'this' pointer but I don't understand this.

Also even it wasn't a non-const function it would be impossible to modify cause of the 'this' pointer that is const. You know what I mean right?

Full names of any object of the class are: this->blahblah. Since this is const it would be impossible anyway right but the compiler just wants to make sure and wont let anything call any function that does not assume this.

Also cant you just use a pointer to modify any of the values without anything going wrong?